In some Mediterranean countries, the "ok" sign is the equivalent of the middle finger.
In Italy (Or, at least Italy several hundreds of years ago), making a fist and sticking the thumb in between the index and middle fingers (called a "fig") is (was?) considered an obscene gesture.
In conclusion, obscene gestures are what you make of them. To someone who knows little about American culture (or nonverbal communication), the middle finger may not be offense at all, because they don't know the true message behind it.